The Java EE 6 Tutorial, Volume I

Using Queries

The RequestBean.adjustOrderDiscount business method updates the discount applied to all orders. It uses the findAllOrders named query, defined in Order:

    query="SELECT o FROM Order o"

The EntityManager.createNamedQuery method is used to run the query. Because the query returns a List of all the orders, the Query.getResultList method is used.

List orders = em.createNamedQuery(

The RequestBean.getTotalPricePerVendor business method returns the total price of all the parts for a particular vendor. It uses a named parameter, id, defined in the named query findTotalVendorPartPricePerVendor defined in VendorPart.

    query="SELECT SUM(vp.price) " +
    "FROM VendorPart vp " +
    "WHERE vp.vendor.vendorId = :id"

When running the query, the Query.setParameter method is used to set the named parameter id to the value of vendorId, the parameter to RequestBean.getTotalPricePerVendor.

return (Double) em.createNamedQuery(
    .setParameter("id", vendorId)

The Query.getSingleResult method is used for this query because the query returns a single value.